Method and apparatus for measuring the index of refraction of thin layers of transparent material



g- 12, 1947' 1.. 'r. SACHTLEBEN 2,42

IETHQD AND APPARATUS FOR MEASURING THE INDEX OF REFRACTION 0F THIN LAYERS OF TRANSPARENT MATERIAL Filed June 17, 1943 2 Sheets-Sheet l g- 12, 1947- L. T. SACHTLEBEN 25,399

METHOD AND APPARATUS FOR MEASURING THE INDEX OF REFRACTION OF THIN LAYERS OF TRANSPARENT MATERIAL Filed June 7. 1943 2 Sheets-Sheet 2 Snnenw:

law" I Ottomgg stances the composition of the evaporatedcoat-- ing is different than that or the original mate- 20 V a 4 Another oblect' oi the invention is to provide it alert-i Au 12,. 1947 METHOD AND APPARATUS FOR 'rna INDEX OF REFRAOTION. or 'rnnv LAYERS F TRANSPARENT MATERIALS 1 Lawrence T. Sachtleben, Indianapolis, Ind., as,-

signor to Radio Corporation of America; 'a'cortporation of Delaware Application June 11, 1943, Serial no. 491,132

9 (llaims.

\ This invention relates to refractometers and more particularly to 'refractometers for the measurement of the index of refraction of thin layers of transparent material. ,For many purposes it is desired to use very .thin layers of transparent material having a thickness ranging from a very small fraction of a wavelength of light up to sometimes as much as several wave lengths. For such purposes the materials are customarily deposited by evaporation in a vacu- 10 um. Even if the index of refraction of the mate-. rial in its normal form is known, it does not necessarily follow that the index of refraction of the evaporated and deposited coating-is the. v same. Depending upon the nature of the maa reiractqmeter for measuring the index of refraction of thintrarisparent films. I

terial, the evaporated coating may have a higher or lower index 01 retraction than theiusedor "-iraictom'eter and method of operation for deter- -mining the "index'ot retraction by the measurecrystalline original material and m. some .in-

rial, which also causes a. change in index of ree traction. It is impractical to measure the indexof 'refraction of such thin films by any of themore. usual methods as practically all reiractom'eters or spectrometers adapted for the measurement of refractive index require a specimen oi'considerable dimension. Even methods involving the use of a microscope are not adapted to meas- I have accordingly devised a novel .type of reiractometer for measuring the indexoi refraction for thin films by a method which is similar which=tana. -(p- '-the index of refraction of the specimen.) g 1 In accordance with the present inventim the matching of. brightnessesoi reflections from films of two different thicknesses which are changing in brightness in opposite directions at the polarizing angle isused to. determine the reaching 01 "that angle. My improved apparatus and method of operation is described in more detail hereinafter.

' One. object or the invention isto provide an improved type of refractomete'r and method 01' operation; a

Another; oblect'i'oi the'inventlon is to provide Another .oliieci-gisto provide an improved rement or B revrsterfs angle.

areiractometer for measuring the index of retirac't'i'on oithin' iilms'to'a-liigher degree of pre-' ision than that-heretofore attained.

which: I

Flgures 1 'and z" are perspectiveviews oithe reiractome'ter constructlonaldetails, and

in some respects to that-described by Blodgett 5 corresponding parts in all the figures.

in the Journal of the American Chemical Society. vol. 5'7, No. 6, June 1935, pages 1007 to 1022, and

in a General Electric Research Laboratory Reprint N0.758.

The Blodgett method consists in observing a .40

specially prepared specimen film in polarized light and determining Brewster's angle of polar-'- ization by viewing the specimen at dlii'erent.

angles. 7

Blodgettmeasures refractive index l n videtermining the polarizing angle by a sensitive method which depends upon interference between the light reflected from the air-film interface and from the film-glass or film-solid interiaceithe.

films being deposited upon the glass): and upon the fact that the. reflected portion or a beam of light plane polarized parallel to the plane of incidence undergoes a phase reversal or change or 130 degrees as the value or this angle of in- The instrument is provided with a base-plate 'III which is "sufilciently massive to maintain the. v instrument in position duringoperation and to maintain the various partsoi theinstrument in their proper relative positions. 0n the base plate is mounted the measuring portion 01' the instrument, which is basically similar to 9. mariners sextant. In fact, a sextant can readily be reconstructed'into an instrument of the present type by the addition and substitution of various parts while retaining the angle measuring mechanism. A I I j p The base-plate carries a graduated are 4!,

cidence passes through the polarizing angle! its travel; A i

. jother-andincidentalobjects of the invention will be apparent tothose skilled in the art irom consideration {of the following "specification in 1 "connection with th'e accompanying drawings,- in

numerals indicate 7 I A collimator tube 1 2 hereinafter described in more detail, is used instead oi the usualsextant telescope, and an optical flat aluminized or silvered on its frontsurface' is providedat l3 in apaxis as the arm l5 and is connected thereto and t the specimen table l6 by the gearing 19, 20, 2|, 22 (see Fig. 2). The gear ratios are $0 chosen that when the specimen l4, whosejplane includes the axis of rotation of the arm ll, is moved a given amount by the arm l5, light from the source l8 reflected from the specimen l4 upon the optical fiat I3 is reflected constantly towardthe optical flatl3. The turntable I6 is rotatably mounted upon the extended spindle oi the hearing oi the arm l which is connected infflxed relation tothe gear 2|. The turntable I6 is secured to'the gear 2| by the screw 32 which permits a relative movement.oifapproximately 90 degrees of the specimen in relationto the arm.

and aifixed gear. The screw30 erves to secure the turntable I6 to the base-plate invany'position within its permitted relative range of rotation of about 90 degrees.

The collimator I2 includes an achromatic objective'23, a plate having .a' narrow slit 24 therein atrlght angles to theplane of-observation and an eye piece-25. A Nicol prism 26' polarizes the light emerging from the .eye piece parallel to the plane of observation, which is parallel to the planeof incidenceoi the light upon the specimen. A plane including the optical axis of the collimator and perpendicular'tothe axis 01 rotation of'the specimen is'aJ-plane of observation. A right angle prism 21 serves to direct the emergent beam of light upward for convenience ln'making observations. The slit 24,:at' rlghtjangles'to the plane of observation, is located at a principal focus of the objective 23v and together with it serves to collimate the light in the plane of observation, thus insuring'flxity or the'direction of observation over the surface of the specimen M in all planes parallel to the plane of observation and polarization. vIn planes perpendicular to the plane of observatiomthe directions of the rays are not so restricted and as a result the I'GSOlVlIlg'POWfil' in these planes exceeds that in those parallel to the plane of observation. The demarcation line in the specimen is set parallel to the plane of observation to take advantage of this greater resolving power and thus show up inthe field of View with maximum sharpness and resolution. If the lineof demarcation between the two different thicknesslayers oi the specimen were set parallel to the slit, its image would be formed at low resolution and the two areas in the field would be separated by a soft or graded boundary rather than separated by a sharp line. 'Theability 0f theeye to detect brightness contrast depends upon the sharpness of the boundary between the two fields of contrasting brightness and the arrangement providing a very sharp boundary line increases theprecision of the observation to a corresponding degree.

A 45 degree unsilvered glass plate 3| isintroduced between the slit 24 and theeye piece 25 permitting the slit to be illuminated from the v rear in the samegeneral manner-as in a Gauss plate 10.

'32 and loosening 30 then prepares the instrument for use after the eye piece 25 has been readjusted to focus upon the image of the specimen All observations using instruments of this type are necessarily taken at angles of observation in excess of 45 degrees. In order to operate from a zero'setting to such angles of observation, the scale ll would have to be at least degrees long although the first half of this scale would never be used in taking observations except for the zero setting. If the specimen is set ahead 45 degrees, it shortens the necessary length 01 the scale II by that amount.

In order to set the specimen ahead 45 degrees, after the adjustment previously described has been accomplished, the arm i5 is moved to the 45 degree position indicated in dashed lines (Fig. 3), the screw 30 is then tightened securing the turntable Hi to the base-plate ill. The screw 32 which secures the turntable I 6 to the arm I5 is then loosened and the arm I5 is set back to zero on the scale ll. The screw 32 is then tightened andthe screw 30 is loosened, leaving the arm I5 and turntable l6 movable together and in a, relative position 45 degrees ahead of the original adjustment.

During the course of these preliminary adjustments, the gear 2| is removed or otherwise dis-' engaged from the remainder of the lamp driving gear train by any suitable means such as that illustrated by Fig. 2. Following these adjustments the gear is replaced or re-engaged, with the lamp on the line of the reflected axis of the collimator, and the instrument is ready for measurements.

' Eifective illumination of the specimen is fur-- thered by the diifusing screen 28 and appropriate thickness to a thickness of A; a wave length over the remaining half of the specimen. The resulting specimen consists of a stepped film of two contiguous portions separated by a very narrow line of demarcation, one thickness of the other.

If the index of refraction of the material to be measured is lower than that of the underlying glass, the wave fronts reflected at angles portion being twice the under the polarizing angle from the air-film and the film-glass surfaces will combine in opposite phase where the specimen is A wave length thick and will combine in the same phase where it is wave length thick. The resultant intensities of the beams reflected from the two portions of the specimen are thus unequal. If the index of refraction of the material is greater than that the two halves of the specimen would exchange intensities. In order to accomplish this result, use is made of the fact that a beam of light plane polarized parallel to the plane of incidence is, for angles of incidence greater than the angle of polarization, reflected in phase opposite to the phase in which itis reflected for angles of incidence less than the polarizing angle. The angle at which this reyersal of intensities occurs is accordingly used to measure the polarizing angle (Brewsters angle) and thereby the inde of refraction.

When such a stepped film is constructed for light of a given wave length and is observed in that light, the light being plane polarized parallel to the plane of incidence, the two steps are seen to be unequal in brightness as just described. If the angle of observation is varied and made to approach the angle of polarization for the film the contrast in brightness between the two steps will be seen to gradually dimi nish until at the polarizing angle the two steps appear equally bright and no contrast is visible at the line of demarcation between them. At this point, the reflectivity at the air boundary of the film for light polarized as described reaches zero. Upon continuing to vary the angle of observation in the same direction, the reflectivity at the air boundary of the film again becomes finite and the two steps again become of unequal and contrasting brightness with the relative brightness of the steps reversed, due to the reversal of the phase of the light reflected at the air-film boundary after passing through the polarizing angle. The high sensitivity of the eye to brightness contrast in conjunction with the reversal of contrast upon passing through the polarizing angle makes this method highly sensitive for determin ing the polarizing angle for a thin film.

In preparing the sample, a wedge-shaped sup-- port should be used so that reflections from the rear surface of the transparent support will not interfere with the measurements. The best result is obtained when the refracting edge of the wedge is set parallel to the plane of observation, and the extraneous reflection cast either upward or downward.

For best results the instrument must be provided with a system of bailles to intercept all but the light which is reflected into the collimator by the specimen itself. This is necessary not only to avoid dilution of the apparent brightness contrast of the specimen, but also to preserve the highly necessary dark' adaptation of the eye which is required by the low brightnesses at which these observations are made. The apparatus, as actually constructed, included a baflle systern consisting of plates which moved with the I claim as my invention:

1. A reiractometer including a rotatable specimen support, a light source rotatable about the same axis as saidlsupport forllluminating said specimen, a stationary observation member including means for polarizing the reflected light image of said specimen .in a plane parallel to the incidence of the light on said specimen, and

means for establishing between said support and said source a rotational speed relation such that a stationary light beam is directed toward said stationary observation member.

2. In a method of determining the index of refraction of one of two-films of the samematerial which are applied to the same. specimen with a boundary line therebetween and have thicknesses such that the thickness of on film differs from'that of the other by one-quarter of the wave length of the light reflected from said specimen, the steps which include rotatingsaid specimen about an axis perpendicularto said boundary line, applying light to said'fllms in a plane parallel with said boundary line and at such angles that light is freflected constantly from said specimen to a flxed point, polarizing.

said reflected light in a plane parallel to said boundary, measuring the angle of incidence at which phase reversal of said reflected light is produced, and using said angle t compute said index.

3. In a method of determiningthe index of refraction of one of two films of the same material which are applied to the same specimen with a boundary line therebetween and have thicknesses such' that the thickness of one film differs from that of the other by one-quarter 'of the wave length of the light reflected from said specimen, the steps which include rotating said specimen about an axis perpendicular to said boundary line, applying light to said films in a plane parallel with said boundary line and at such angles that light is reflected constantly from said specimen to a fixed point, polarizing said reflected light in a plane parallel to said boundary and restricting said reflected light along a line perpendicular to said boundary, measuring the angle of incidence at which phase reversal of said reflected light is produced, and using said specimen for applying light to said specimen in a plan parallel to said boundary and at such angles that the light beam reflected from said specimen is stationary, and means for polarizing said light in a plane parallel to said boundary.

5. In' a device which functions to present to the observer a contrast in the intensities of the lamp house itself, and plates which were flxedto beams reflected'from the two parts of the stepped surface of a specimen, the combination of means for rotating said specimen about an axis which is perpendicular to the boundary between said parts, means rotated simultaneously with said specimen for applying light to said specimen in a plane parallel to said boundary and at such angles that the light beam reflected from said specimen is stationary, means for polarizing said reflected light in a plane parallel to the plane of incidence of the light upon said specimen, and a light slit interposed in the path of said reflected ht, w th. its

: 7 v light with its length dimension perpendicular to said boundary.

6. The combination of means :ior supporting a specimen having parallel plane surfaces separated by a boundary line-means for rotating said specimen about an axis perpendicular to said boundary line, means rotated simultaneously with said specimen for applying light to said specimen in a plane parallel to said boundary line and at angles such that the reflected lightbeam is stationary, and means for polarizin said reflected light in a plane parallel to said boundary.

' 7. The combination or a specimen support table,' means for rotating said table about an axis perpendicular to its specimen supporting surface, means rotated simultaneously with said specimen for applying light to a specimen in a plane parallel with said surface and at angles such that the light beam reflected from saidspeclmen is stationary, andmeans for polarizing said light inaa; plane parallel with-said surface 8; The combination of a specimen support table; means for rotating said table about an axis perpendicular to its specimenv supporting surface, means rotated simultaneously with said specimen for applying" light 'toa specimen-in a plane parallel with saldjsurlace and at angles such that'the lightbearnreflected froinsaid specimenis stationarypmeans for polarizing said re- 'fiected light-'in ffa plane parallel to the plane of incidence oi' -thelight upo'nsaidispecimen, and a light slit'inter' f ied in the patlirof said reflected said'suri'ace';

th dimensionjperpendicular to specimen support Number table, ineans for rotating said table about-an axis saidreflected light passes for-establishing a zerotable. LAWRENCE T. SACHTLEBEN. REFERENCES CITED:

The following references are of record in the file of this patent: I

FOREIGN PATENTS I setting of said Country v Date 97,515 Germany June 8, 1898 483,173 France Mar. 10,1917

7 OTHER REFERENCES The Gloss Characteristics of Photographic Papers, an article by Jones et al. in British Journal of Photography for April 14, 1922. Pages-216- I 218 cited. (Copy in Scientific Library, U. Sf fPatent Oflice.) (88-14ZR.)

Physics for Technical'studentsfl'by Anderson, pub. 1921; page 417 cited. (Copy in Div. 7,

U. S. Patent Oflice.) 

